This invention relates to ferroresonant transformers such as those used in power regulation, and especially to the use of such ferroresonant transformers as self-regulating power control devices. More particularly, the invention relates to the use of ferroresonant transformers in systems where more than one load is to be powered, and where the loads are preferably electrically and magnetically isolated from one another.
Ferroresonant transformers have been used in many applications, including voltage regulating systems, for several decades. They comprise basically a laminated steel core around which are wound separate primary and secondary windings, with steel shunts placed between the primary and secondary windings. These magnetic shunts between the primary and secondary windings create an inductive coupling between the primary and secondary circuits. Integral with the secondary winding is a resonant winding coupled to a capacitor, sometimes called a "ferrocapacitor." The capacitor, or ferroresonating capacitor, shunts the saturating inductor or winding, and is usually near resonance with the linear inductance.
The combination of the resonant capacitor and the inductive coupling produced by the shunts creates a resonant circuit. The gain of this resonant circuit drives the magnetic flux in a portion of the core within the secondary winding to saturation. That is to say, this portion of the core cannot be driven to a higher flux density despite changes in the input voltage or output load. Since voltage induced in the secondary winding is proportional to flux density, the voltage at the terminals of the secondary winding (the load voltage) remains constant.
The ferroresonant transformer thus functions to provide a constant output voltage despite changes in output load or input voltage. In addition, the saturation of the secondary section of the core causes the output waveform to be nearly a square wave rather than a sine wave. This is advantageous where the output is rectified and filtered in order to provide a D.C. power supply.
An additional advantage of the ferroresonant transformer is that the inductive coupling of the primary and secondary circuits makes the transformer inherently current-limited. If the secondary is shorted, the primary current is limited to safe levels because there is, in effect, a substantial inductance between the primary and secondary circuits.
There are numerous applications for ferroresonant transformers where multiple loads are to be powered, and it is desired to provide redundancy such that the short circuit of load will affect the others. This is conventionally accomplished by using multiple transformers.
An example of this is in cable television applications, where feroresonant power supplies are used to provide 60 volts A.C. on the distribution cable to drive amplifiers and other components. It is desirable to isolate sections of cable from one another so that a fault on one section which shorts the cable will not affect adjoining sections of the cable.
The solution, as indicated above, has been to use two or more ferroresonant transformers to achieve the desired electrical and magnetic isolation between the different sections. This is a cumbersome and costly arrangement, and is particularly undesirable where weight constraints are in the picture.
The device of the present invention reduces the difficulties indicated above, and affords other features and advantages heretofore not obtainable.